The Internet is essentially a network of interconnected queues. The queues are maintained at the ports of the various routers, switches and servers that form the links of the communications network that is the Internet. The network equipment, or nodes, maintain the packet queues using queue management processes executed by the equipment. Packets traversing the network experience delay and loss. The Internet is facing increasing packet loss rates and queuing delays. Lost packets waste resources, whereas queuing causes packet delay which reduces the quality of interactive applications.
Active queue management (AQM) processes have been introduced to alleviate the problems of network congestion. In general, AQM processes control congestion by controlling flow. Congestion is measured and a control action is taken. AQMs apply a congestion notification rate, using packet marking or dropping, to control the effective ‘price’ of bandwidth detected at a packet source so that the arrival rate for a link is at or below the link's capacity. An AQM can be conceptualised as generating a resource ‘price’ determination at the link and the source then follows a utility process that determines its consumption of bandwidth based on the resource ‘price’.
There have been two approaches for measuring congestion: (1) queue based, and (2) flow based. In queue based AQMs congestion is observed by queue size. The drawback of this is that a backlog of packets is inherent in the control mechanism, as congestion is observed when the queue is already positive. This creates unnecessary delay and jitter.
The total network delay is essentially the sum of queuing delay, in routers and switches and other equipment of the network, and propagation delay in the physical transport medium, such as the optical fibre or copper wire that support the links. Currently queuing delay dominates most round trip times (RTT), being the time a packet takes to travel from a source to a destination and return. An ideal is to reduce the network delay to just the propagation delay.
Currently packet loss is both a signal of congestion and result of overflowing queues. The Internet Engineering Task Force (IETF) is introducing explicit congestion notification (ECN) to feedback congestion by packet marking instead, as discussed in RFC 2481. Congestion notification itself may be taken to involve either packet dropping or ECN. With ECN, AQM queues can operate with minimal packet loss.
It is desired to provide an AQM process that maintains high link utilisation whilst achieving the QoS (Quality of Service) requirements of limited packet loss and delay, or at least provide a useful alternative.